Studies of genetic alterations that contribute to tumor development have focused almost exclusively on protein coding genes. It is now clear that the genome generates a variety of non-coding RNAs that have widespread and important roles in gene regulation. The microRNAs are an abundant class of small RNAs that modulate mRNA expression and stability through the RNAi pathway. A number of these genes have been implicated as oncogenes and tumor suppressors. In this Project, we propose to investigate the biological function on a bona fide microRNA oncogene, mir-17-92. The ultimate goal of Project 5 is to understand the precise mechanism through which mir-17-92 contributes to tumor development. This will be accomplished through structure-function studies of the microRNA polycistron and through the identification of the mRNA targets of its key oncogenic components. Additionally, we will probe the normal biological function of this oncogene through the generation and characterization of both gain-of-function and loss-of- function mutants. Finally, we will explore more broadly the role of the microRNA machinery in tumor development through genetic manipulation of the microRNA biogenesis machinery and through a search for microRNAs that show expression changes in breast carcinoma, hepatocellular carcinoma and B-cell lymphoma.